A plasma is an ionized gas, and is usually considered to be a distinct phase of matter. “Ionized” in this case means that at least one electron has been dissociated from a proportion of the atoms or molecules. The free electric charges make the plasma electrically conductive so that it couples strongly to electromagnetic fields. The term plasma is generally reserved for a system of charged particles large enough to behave as one. Even a partially ionized gas in which as little as 1% of the particles are ionized can have the characteristics of a plasma (i.e. respond to magnetic fields and be highly electrically conductive).
The use of larger atmospheric plasmas in chemical analysis is well established, with inductively coupled plasmas (ICP) and microwave cavities offering ionized sources for Optical Emission Spectroscopy (OES) or Mass Spectrometry (MS). In these established systems, input powers of greater than 1 kilowatt (KW) and gas flows of greater than 10 L/min are employed to maintain stable plasma conditions at atmospheric pressure. The gas flow is made up from a gaseous or aerosolized analyte and a make-up gas (e.g., He or Ar diluent). At such high flow rates, the make-up gas is the majority of the gaseous flow, with the analyte component often just a few mL/min. Hence large dilution factors are common which can adversely affect detection sensitivity.
Reducing the size of the plasma to the microplasma regime (e.g., nL−μL volume) offers the opportunity to reduce both input power and gas flow rate. A review of microplasmas for chemical detection can be found in Karanassios, et al. (V2004 Microplasmas for Chemical Analysis: analytical tools or research toys. Spectrochim Acta B59:909-28). FIG. 1 (below) provides exemplary depictions of prior art microplasma devices.
FIG. 1A provides an exemplary depiction of a prior art microplasma device employed for analyte detection applications. In FIG. 1A, sample ionizing device 10 contains a plasma generation source 14, a sample feed 26, a gas feed 16, and a sample input port 18. In this device, the sample feed 26 directs the sample (e.g., gas containing analyte of interest) into the gas feed 16 (in direction of dotted arrow marked S) where it is diluted in the gas flowing in the gas feed 16 (in direction of dotted arrow marked G) and delivered to the plasma generation source 14 which produces a plasma plume 12 that contains ionized sample. The injection of a sample feed into a gas feed is similar to that of larger scale ICP torch systems. However, the macro size of these systems allows the sample feed to be run coaxially with the much larger diameter gas feed, instead of orthogonal as shown in this microplasma example. The plasma plume 12 exits the plasma generation source 14 into the air. Analytes ionized in the plasma may then be detected, e.g., by using optical emission spectroscopy, or mass spectrometry. Similar to the use of larger ICP torch devices summarized above, as the plume exits into the atmosphere it interacts with the surrounding air leading to contamination of the plasma because non-sample analytes from the atmosphere are ionized in the plasma. Increasing the power and gas flow rates can reduce the diffusion front of air (and its associated contaminants) with the main body of the plasma, but cannot eliminate it completely. Indeed, for large ICP torch systems the environment into which the torch emits needs to be controlled in order to bring chemical sensitivity down to the parts-per-trillion (ppt) level.
FIG. 1B demonstrates one prior art way to improve the functional parameters of microplasma-based sample ionizing devices. In this example, the plasma plume 12 is enclosed in a housing 20 such that the overpressure of the incoming gas flow prevents air diffusion into the small plasma volume (direction of the gas flow is indicated by dotted arrows G and E). Here again, however, moderate to high gas flow rates are required to prevent air diffusion (and thus contamination) from the top opening of the housing (also known as the exit port). While the length and diameter of the exit port can be altered to prevent back diffusion of air into the plasma and thus function with a lower flow rate (e.g., making the housing longer and narrower), this can produce high back pressures and long residence times in the plasma plume 12. If the plasma is used as a detection source for gas chromatography, for example, long residence times for the analyte will lead to trailing peaks in the chromatogram.
As such, there is continued interest in the development of new microplasma-based devices and systems, e.g., that can be employed for high sensitivity analyte detection.